How we remember
| May 1, 2008
In-Depth Report
How we remember
You just saw a new film, and stored it — along with other information and events that you encountered today — in your brain. But where, exactly, did it go? Is your brain's system for storing memories a "memory bank," a single repository of all the sights, sounds, and facts that have made a strong enough impression for you to remember them? Or is it a kind of library, with different memories categorized by something akin to the Dewey decimal system, and then stored in different "stacks" from which they can be retrieved?
As good as memories in the bank
One of the enduring myths about memories is that they are kept in one place in the brain, a memory bank. But over many decades of research, scientists realized that this assumption was wrong. In the 1980s, when functional brain-imaging technology — such as positron emission tomography (PET) scans — became available, scientists could see people's brains at work for the first time (see "Watching the brain at work"). They could obtain images of the brain as people performed a variety of tasks, including remembering things. Researchers discovered that memories are not stored in a single location, but rather are widely distributed in networks throughout the brain, primarily in the cerebral cortex. The cerebral cortex consists of the outer covering of the two large hemispheres of the brain and is the most highly developed part of the human nervous system. The cortex contains about 20 billion neurons that collectively function to integrate sensory information, control voluntary movements, and mediate thinking processes.
Different areas of the brain process different kinds of information. For example, auditory information, including speech and other sounds, is processed in the temporal lobes (see Figure 2), while the registration of visual images occurs in the occipital lobe at the back of the brain. What these findings suggest is that a particular aspect of a memory will most likely be stored in a region of the cortex that specializes in processing similar information.
Therefore, the words of "The Star-Spangled Banner" would be stored in the language regions of the left temporal lobe, but the melody would be stored in different regions of the brain's auditory cortex. And each memory is connected to many related memories. For instance, if you associate "The Star-Spangled Banner" with the image of the American flag, that memory might be stored in your occipital lobe, which processes visual information. Your memories are thus intricately broken down and cross-referenced, making your brain less like the shelves of a library and more like the Internet. Calling up memories is like doing an Internet search, with one or two words activating many hyperlinks.
But just how does the information that you encounter on a daily basis get filed away? Memory researchers use a three-stage model to describe how the brain learns and remembers each bit of information: acquisition, consolidation, and retrieval. Impairment in any one of these three stages can result in a failure of memory.
How smoothly the process goes — which is to say, how well you remember — depends on many things. Genetics plays a major role. Studies of human intelligence suggest that approximately 50% of mental ability is genetically determined. It may be that some people are simply better at remembering than others because of a genetic predisposition for an excellent memory. But factors within your control are also important. Overall physical health, emotional well-being, stress level, sleep quality, and diet exert a huge influence on how well you learn and remember.
Figure 2: Brain map
Different parts of the brain specialize in different functions. Memories are stored in many parts of the brain but some areas are more critical than others. To retrieve a memory, these areas of the brain must work in coordination with others. For example, the frontal lobe (important for planning and organization) works together with several brain regions in acquiring and retrieving memories. |
Stage one: Acquisition
It's common sense: to remember anything, you must first learn it. When new information is learned or acquired, it first takes the form of temporary pathways of nerve cell activity in the brain, as one neuron communicates with the next. The location of these neuronal pathways depends on the nature of the information. For example, activities such as speaking and writing activate neurons in the left temporal lobe, which processes language for most people, whereas studying a map activates neurons in the right parietal lobe, which processes spatial information (see Figure 2).
The neuronal activity that represents the information you've just learned is temporary, and the new information is part of your short-term memory. Most of this information will quickly fade away. The memories that endure will be those that were encoded most completely in the first place — the information that you paid the closest attention to when you learned it. When you have trouble remembering a piece of information, it's often because you weren't paying close attention when you initially encountered it. One of the reasons older people have more trouble remembering things is that they are more easily distracted by background noises and other interruptions, which can interfere with initial learning. Memories that involve multiple senses as well as emotions are more likely to be retained.
Stage two: Consolidation
Let's say that you were paying close attention to the information you just heard and that it was effectively encoded in your brain. How does it become stored as a memory that you can recall in the future? For the information to become a long-term memory, its initial neuronal pathways must be strengthened. The strengthening process, typically referred to as consolidation, occurs over a period of time.
Consolidation of declarative memories
The hippocampus plays a major role in consolidating declarative memories. Several factors influence whether the hippocampus responds to the newly acquired information and gives the signal to store it as long-term memory. For example, you're more likely to retain new information if it relates to long-term memories you already have. It's easier to remember the names of the players on your local football team if you know something about football than if you don't follow the game at all. Another factor is the information's emotional impact (see Figure 3). You're far more likely to remember a disturbing photograph than a bland one. You can probably remember the images of the World Trade Center falling far more vividly than you can, say, a photo of a hotel where you attended a business conference. The part of the brain that reacts to emotionally powerful information is the amygdala, which is situated near the hippocampus. Some research using PET scans suggests information that activates the amygdala is most likely to be retained long-term.
Figure 3: Remember grandma's cookies?An experience that stimulates many parts of the brain is more likely to be remembered, particularly if it has an emotional component.
|
Consolidation of procedural memories
In contrast to declarative memory, the consolidation of procedural memories is less dependent on the hippocampus. Even people with amnesia, who have damage to the hippocampus, can form new procedural memories — that is to say, they can learn new skills through practice. Procedural memories, such as riding a bicycle or playing a musical instrument, also tend to remain intact with aging. They even persist during the early stages of Alzheimer's disease, which damages the hippocampus. However, acquiring new procedural memories becomes more difficult with age because we process information more slowly.
Procedural memory is stored throughout the brain in regions that are important for coordinating movement or sequential processing, such as the frontal lobes, the cerebellum, and the basal ganglia. Studies also show that sleep is vital for consolidating procedural memory. In an experiment at Harvard University, students who learned a computer game were better able to remember it and play it the next day if they'd had more than six hours of sleep than if they'd had less. Even two days to a week later, students who were consistently well-rested outperformed those who didn't sleep as well.
Stage three: Retrieval
Retrieval is the act of recalling something. A memory is stored in the brain as a unique pattern of nerve cell activation. When you're not thinking about the memory, its neuronal pattern is inactive. To retrieve the information, your brain must reactivate the pattern. Similar memories have partially overlapping patterns of neuronal activation. Sometimes when you try to retrieve one bit of information, a similar memory comes to mind and blocks out the information you want. For example, you may be trying to recall the name of Tom Hanks' first movie, but instead you keep thinking of the name of his most recent film.
Researchers have determined how long it takes to reactivate a neuronal pathway holding simple, familiar information — less than a second. They've found, for example, that when someone sees a photograph and is asked whether it's familiar, it takes about a fifth of a second for the image to reach the visual system in the brain, a fifth of a second for the person to decide whether it's familiar, and another fifth of a second for the person to reply.
If it always took just a fraction of a second to remember something, you wouldn't worry about your memory. But, of course, it often takes considerably longer. Even if your memory is perfect, it can take several seconds or more to recall complicated information. How long the process takes depends on how familiar you are with the information you're looking for. If someone asks you to name the third president of the United States, for example, and you're an American history teacher, you may be able to recall in an instant that the answer is Thomas Jefferson. Otherwise, your brain will activate neuronal pathways that encode information related to the chronology of the presidents — the year when the United States was founded and the names of any early presidents that come to mind, for instance. In the process, you may feel that the answer is "on the tip of your tongue." If the neuronal pathway in your brain leading to the answer is still intact, you'll eventually retrieve it.
Review Date: 2008-05-01
Harvard Medical School does not endorse products or services.
preview